Glass compositions and glass-to-metal seals



Aug, 1960 F. OOLDFIELD ET AL 2,948,992

GLASS COMPOSITIONS AND GLASS-TO-METAL SEALS Filed Feb. 21, 1958 INVENT K j flea/e? p/n) United States Patent 2,948,992 GLASS COMPOSITIONS AND GLASS-TO-METAL SEALS Lucy Florence Oldfield, Harrow Weald, England, and

John Henry Partridge, deceased, late of Pinner, England, by Ethel Partridge and Michael John Anthony Partridge, administrators, Pinner, England, assignors to The General Electric Company Limited, London, England, a British company Filed Feb. 21, 1958, Ser. No, 716,561 Claims priority, application Great Britain Feb. 21, 1957 8 Claims. (Cl. 49-925) to glass-to-metal seals formed by sealing glasses in accordance with the invention to nickel or to other metals and alloys of similar thermal expansion characteristics as aforesaid.

Nickel, and such other metals and alloys, may be employed for the construction of components of some electrical devices: for example, nickel may be used for forming a part of the envelope of a device, or forleadingin conductors. Hence, in the manufacture of such a device, it may be required to seal one or more nickel components, or components of other metals and alloys as aforesaid, to a suitable glass, and the invention further relates to such electrical devices in which the glassto-metal seals are manufactured using a glass in accordance with the invention.

It is well known that in order to obtain a satisfactory glass-to-metal seal, with little or no stress in the glass in the vicinity of the seal, it is desirable that the thermal expansion characteristics of the glass and metal components to be sealed together should be closely matched'to one another over the range of temperatures to which the seal is subjected in manufacture thereof and in the manufacture and operation of the device of which the seal forms part: close matching is particularly desirable at the upper annealing temperature of the glass.

The present invention is. concerned primarily with the provision of glasses capable of sealing satisfactorily to nickel, the thermal expansion coefiicients (u)'of which over various temperature ranges are as follows:

Over the temperature range of 20 C. to 350 C.;

a= 15.0; :0.1 10- cm./cm./ C.

Over the temperature range of 20 C. to 400 C.;

Over the temperature range of 20 C. to 450 C.;

a=15.3i0.1 cm./cm./ C. Over the temperature range of 20 C. to 500 'C.;

a=15.4i0.1 cm./cm./ C.

Thus it is" an object of the present invention to provide a sufliciently high thermal expansion lcoeflicicnt to. render 2,948,992 Patented Aug. 16, 19 0 ICC differ from that of nickel by substantially more than 0.5 10- cm./cm./ C., at the annealing temperature of the glass. The glasses in accordance with the invention will in general seal satisfactorily to other metals and alloys having thermal expansion characteristics similar to that of nickel, as aforesaid. p i

It is furthermore desirable for glasses used in the construction of electrical devices to possess high electrical resistivity, for example not less than 10 ohm cm. at 50 C., so that the glasses are effective as insulators; it is also desirable for glasses so employed to be resistant, to attack by moisture and by chemicals, especially dilute mineral acids, which may be used for cleaning the metal components of the devices, and to withstand heating to relaincluding only 0 to 5% calcium oxide, 0 to 2% fluorine (F 0 to 2% alumina (A1 0 0.5% to 10% zinc oxide (ZnO), and 0.5% to 3% of either zirconia (ZrO or titania (TiO the total content of alkali metal oxides (Na O+K O) being in the range of 18% to 22%.

According to another aspect of the invention, a glassto-metal seal consists of a metal body of nickel, orother metal or alloy of like thermal expansion characteristic, sealed to a glass having a composition within the range specified in the preceding paragraph; 7

According to yet another aspect of the invention, an electrical device has an envelope including or containing at least one glass-to-metal seal of the kind specified in the immediately preceding paragraph.

All proportions referred to in this specification and in the appended claims are by weight, and the range of compositions referred to above for the glasses in accordance with the invention is inclusive of the end figures quoted for each constituent. V

The compositions specified herein are the compositions of the glasses after founding and for arriving at them account might need to be taken, in preparing the initial batch of raw materials, of possible changes'in composition which may occur during founding due to the volatilisation of part of some constituent, especially fluorine and alkali metal oxides, and to corrosion of the refractory material of which the furnace walls are formed, which may result, for example, in an increase in the alumina content of the glass by an amount up to 1%. Thus it may be necessary to adjust the composition of the initial batch, in Well known manner, by including therein slightly larger proportions of the ingredients yielding fluorine and alkali metal oxides, and a slightly smaller proportion of alumina, than the proportions theoretically required to give a glass, of the .desired composition.

However, by carrying out the founding under carefully controlled temperature conditions, and in a pot or tank whose walls are composed of material not subject to appreciable corrosion by the molten glass, it is possible to produce a glass whose analytical composition is very close to the nominal composition, that is to say the exact composition aimed at, with little adjustment of the batch composition.

The glasses in accordance with the invention are manufactured in well-known manner by melting batches of raw materials'commonly used in'the glass manufacturing industry for providing the various constituents of the glasses, specified above; For example, in addition to sand the batch usually includes the carbonates of the alkali metals and alkaline earth metals, and the oxides of zinc and either zirconium or titanium, and may also contain alumina. Cryolite is preferably used as the source of fluorine, when the latter is included, and also provides a small proportion of alumina. The glasses may be founded at temperatures of 1300 C. to 1350 C., suitably in a tank furnace having walls ofaluminosilicate refractory material; preferably the furnace walls are composed of mullite tank blocks manufactured by the method described in US Patent No. 2,675,324. Founding is completed in a relatively short time, for example two hours from the final addition of batch being sufficient for a quantity of glass weighting about 1 /2 hundredweight. Such a quantity of glass is cooled to the working temperature, which for these glasses is about 1000 C. to 1050 C., in about two hours.

All of the glasses in accordance with the invention which we have prepared and examined are of the required degree of softness specified above, annealing at temperatures within the range of 400 C. to 500 C., and having Mg points in the range of 470 C. to 510 C.:'the Mg point of a glass is defined as the highest temperature attainable on the thermal expansion-temperature curve obtained with a rod of the glass, above which temperature the glass is deformed, at a rate similar to its rate of expansion, by the small pressure exerted on the rod by the optical lever or dial gauge against which it abuts in the apparatus used for determining the expansion, the rod usually being supported horizontally if an optical lever is used and vertically if a dial gauge is used. All of said glasses also possess coefiicients of thermal expansion in the range of' 14.5 to 15.5 lcm./crn./ C.

at their upper annealing temperatures, and are therefore sufiiciently well matched to nickel to form satisfactory seals therewith and with other metals and alloys of similar thermal expansion characteristics to that of nickel. Furthermore, we have found that these glasses are capable of forming satisfactory mismatch compression seals with metals of higher thermal expansion coefficients than nickel, for example copper. The high thermal expansion of the glasses of the invention is achieved by using higr proportions of alkali and alkaline earth metal oxides.

These glasses are further characterised by possessing a high degree of fluidity, being capable of flowing readily at temperatures of 700 C. to 800 C. An indication of the fluidity of a glass is given by its fibre softening point, which is defined as the temperature at which the viscosity of the glass is poises. The glasses of the invention possess low fibre softening points, those which we have determined all being in the range of 600 C. to 650 C. The fluidity of the glasses is increased by the inclusion of a small proportion of fluorine, within the range specified above.

The inclusion of the specified proportions of the oxides of the divalent metals strontium, barium and zinc ensures that the glasses will possess relatively high electrical resistivities, above 10 ohm cm. at 50 C., the said divalent metal oxides counteracting the effect of the high alkali content which tends to reduce the resistivity as compared with glasses of low alkali content.

The glasses of the invention possess sufliciently good 4 chemical durability to enable them to resist attack by the acids generally used for cleaning nickel, in the manufacture of devices which include nickel components; the required chemical durability is achieved by using the correct relative proportions of the alkali and alkaline earth metal oxides, and by the inclusion of alumina, zirconia or titania, and zinc oxide, which improve the chemical durability of glasses.

The tests employed for determining the durability of these glasses consist essentially in boiling two samples of the glass, for one hour respectively in distilled water and in dilute hydrochloric acid containing 20 ml. of HCl in ml. of the solution. The samples are wiped dry gently with paper tissues, heated in air to a temperature below 80 C., and examined under a low power microscope for surface leaching and other defects. The samples are then heated at C. for 30 minutes, and re-examined. A glass so treated is considered to possess good durability if both of the samples show only slight pitting or crazing, to a penetration of less than one thousandth of an inch, visible at a magnification of 10, with no surface breakdown, and solution of less than 0.5% by weight of the glass. If negligible pitting or crazing is apparent at a magnification of 10, although these effects may be detectable at a magnification of 30, and solution of the glass is negligible, that is to say not more than a few milligrams dissolved from 10 grams of glass, the durability of the glass is regarded as excellent. The examination of the samples is carried out on freshly broken surfaces, which are less durable than fire polished surfaces; in some cases fire polished surfaces are also examined since the durability of such surfaces is of interest in relation to fabricated glassware.

The glasses of the invention are free from easily reducible oxides, such as lead oxide: this feature of the glass composition is particularly advantageous when the glass is used for manufacturing articles by sintering glass powder initially mixed with an organic binder, as for example in the manufacture of sintered glass beads for sealing leading-in wires into electrical devices. Thus there is no tendency for reduction of any of the constituents of the glass by the organic binder or any decomposition product thereof to occur, so that a clear colourless glass may readily be obtained.

It may be noted that, although the total alkaline earth (BaO, SrO and CaO) content of these glasses is relatively high, from 20% to 30%, the calcium oxide content is not greater than 5% this low limit on the calcium oxide content reduces any tendency of the glasses to devitrify, so that they are substantially free from devitrification at the working temperatures. This property may also be of value for a glass incorporated in an electrical device may be subjected to temperatures up to 1000 C. in manufacture or operation.

The glasses in accordance with the invention possess the additional advantages that they are easily founded, and we have found that very little variation occurs in the resultant composition of the products obtained from different raw batches each of the same composition, and that they are of good quality suitable for use in the manufacture of tubing, envelopes of electrical devices, and similar ware, by well-known methods.

The compositions and some of the properties of some specific glasses, A, B, C, D, E, F and G, in accordance 'with the invention are given in the following table, by way of example. The compositions are in parts by weight, and the properties referred to are the mean coeflicients of thermal expansion (at) over the temperature range from room temperature to the Mg temperature, the Mg temperatures, annealing temperature ranges,

the electrical resistivity (p) expressed as the value of the logarithm to base 10, the fibre softening points and the chemical durability.

Table Composition A B C D E F G 810 48.0-- 47.0.. 50 0 50.0. 48.0.. 43. 0 48.0. N320- 21.0-- 20.5-- 13.0-- 13.0- 20.5-- 21.0 21.5. K20 8.0-- 8.0 380.. 8.5.. 9 "I 8.0-- 8.0- 9.5.- 8.5-.- 8.0. SrO 10.0-- 10 0-- 10.0-- 10.0- 10.0-- 10.0 10.0. CaO- 3.0--- 4.0.. 3.0- 3.0 3.0--- 3.0.. 3.0. F 1.5-- 1.5- 2.0 2.0 2.0. A150, 1.0- 1.0--- 1.0-. 1.0. 1.0- 1.0- 1.0. 2110---- 5.0.-- 5.0--- 5.0.- 5.0-- 5.0--- 0.0-- 5.0. ZrO 2.0-- 1.5-- 2.0 2.0 1.0 2.0. T10 2.0 Properties:

a 10 era/em 1C C. 15.2 14.9.- 14.5 14.7 15.0 14.6 14.5. Mg 0 G" 5 C 470 425 505 505 475 485. 500. Annealingrange,0--. 450-400 465-415 485-435 485-435 455-405 465-415 480-430. Fibre softe O.:l=5 605 610 640.-- 540- 005 000- 540. log 109, ohm cm at 10.9 11.0 14.5 14.6 11.0 Durability:

Water Test Excellent.. Excellent... Excellent... Excellent... Excellent... Excellent... Excellent. HO1Test Fairly good. Fairly good. Good Good Fairly good. Fairly good. Fairly good.

One specific method of manufacturing a glass in accordheating the bead and metal components to a suitable temance with the invention will now be described by way of perature above 700 C. example. In many cases the most convenient type of bead to use In this example, for the production of approximately is that formed of sintered glass powder. "100 lbs. of glass of composition A in .the above table, Glass powder, for use either in the loose state or for a typical batch of raw materials is prepared by mixing forming sintered glass beads, can be produced from any rthe following ingredients: of the glasses with which the invention is concerned by lDutch Sand (sioz) 481bs dragading, that is to say by pouring the molten glass into water to 01m coarse granular material from which the witherite (BaCO 11 lbs. water is immediately removed by siphoning and which is .Limespar (CaCO 5 lbs., 6 ozs. O o then dried at 120 C. to 200 C., the dragaded glass is .Strontium carbonate 14 1bs.,4ozs. then ball milled to reduce it to powder mainly consisting .Sodium carbonate 36lbs. \Cryofite (3NaF AIF 4113s 10 0Z8 of particles capable of passing through a sieve having 40 3 meshes to the linear inch but retained on a sieve having Z1110 made 5 lbs. 00 h h h Zirconia zlbs. 2 mes es tot e 111631 inc For forming sintered glass beads, for example from The amount of cryolite included in this batch is sufliglass A in the above table, glass powder produced as cient to allow for 25% loss of fluorine during founding. I 4 described above is mixed with a binder consisting of The alumina is also derived from the cryolite, together 40 polybutyl methacrylate, 100 grams of glass powder being with a small amount picked up from the furnace refracmixed with 3 to 5 grams of polybutyl methacrylate, distory. The sodium content of the cryolite corresponds solved in 25 cc. of sulphur-free toluene; the mixture is very closely to the amount of Na O volatilised during dried in air at-approximately 40 C. for at least 24 hours, founding. Y with occasional stirring to prevent making. The dried, The batch is introduced, in several portions successively, bonded powder is granulated through a 40 mesh sieve, into a small tank furnace, having walls composed of and is then compacted under a pressure of 4 to 5 tons mullite-type blocks. The material in the tank is heated per square inch in a mould to form beads of the reto 1300 C. and is maintained at this temperature for quired size and shape and provided with apertures for two hours after the final addition of batch, to complete 1 the insertion of conductors, if required. the founding of the glass. The glass is allowed to cool The compacted beads are heated in stages to effect first in the tank over a period of 2 hours to the working volatilisation of the binder, and then sintering. The temperature of 1050 C. v beads are first heated slowly in air to 200 C. to 250 C. The glasses of the invention, exemplified by the comand are maintained at this temperature for /2 to 1 hour positions given-in the above table, are suitable for the to ensure complete removal of the binder. The temperamanufa ture f glass ea s Which m y used, for exture is then raised slowly to 550 C. to 560 C.; thebeads ample, for sealing small components of nickel or other are maintained at 550 C. to 5 C. for about 10 minutes metal or alloy of like expansion characteristics such as to effect sintering. The beads are allowed to cool in the wires or small cylinders, into components formed of other oven in which the heating has been carried out, the high expansion metals, such as copper canisters. I oven door being left slightly open to prevent over-sinter- The glasses may also be employed in powder form for 60 ing. I sealing two metal components together, the loose glass One form of glass-to-metal seal in which a glass in acpowder being introduced without a binder between the cordance with the invention is employed, and an electrisurfaces of the metal components to be united and the cal device incorporating such a seal, are shown in the whole assembly being heated to a suitable temperature accompanying drawings, and will now be described by as aforesaid with or without an auxiliary pressing operaway of example. In the drawings, tion. One of the most important uses of these glasses is, Figure 1 shows a glass-to-metal seal in sectional elein the form of glass beads, for sealing leading-in conducvation, and tors into apertures in metal components: the beads may Figure 2 is a perspective view of a device incorporating be formed by any of the well known techniques which a seal of the form shown in Figure 1. comprise heating a short length of glass tubing slipped Referring to Figure 1 of the drawings, a nickel tube over the conductor, or sintering glass powder, or apply- 1 is sealed into the aperture in a re-entrant part 3 of the ing the softened end of a glass rod to the surface of a conend wall of a copper cylinder 2, using -a bead 4 which ductor and rotating the rod and the conductor relative is pre-formed by sintering a powdered glass in accordance to one another until a bead of glass has been deposited with the invention. around the conductor; in each case the seal is formed by The device shown in perspective in Figure 2 of the drawings, which may for example be a semi-conductor device of known kind, comprises a cylindrical copper canister containing any desired operative components (not shown in the drawing). The copper canister is. formed of two parts 5 and 6, provided respectively with flanges 7 and 8 which are united together, for example by coldv Welding. The end surface 9 of the part 5'has a central re-entrant apertured portion, indicated at 10, into which are sealed a glass bead 11 and a nickel tube 12, the seal being of the form shown in, and described above with reference to Figure 1.

The glasses of the invention may also be employed for the manufacture of tubing, and ware such as the glass parts of envelopes of electric discharge devices, by Well known drawing, blowing and moulding techniques. Such glass components can be sealed directly to components formed of nickel or a metal or alloy of like thermal expansion characteristic, by heating contacting parts of the glass and metal components to a suitable temperature between 700 C. and 800 C. at which the glass flows readily.

In order to obtain a strong vacuum-tight seal between a glass in accordance with the invention and nickel, it is desirable that the surface of the nickel should be treated to produce a film of green nickelous oxide thereon, immediately prior to the sealing operation. In the preferred method of treating nickel for this purpose the nickel components are first immersed in a cold solution consisting of 900 ccs. of distilled water, 750 ccs. of concentrated sulphuric acid, 1000 ccs. of concentrated nitric acid and 50 gms. of sodium chloride. The nickel is left in this solution until gas is being evolved freely, usually not longer than 30 seconds, and is then washed thoroughly with distilled water, the last traces of acid being removed preferably by boiling in distilled water. The nickel is rough dried, and then heated at 1050 C. in wet hydrogen for 30 minutes. The nickel is finally heated in a reducing flame to approximately 1000. C. for a few seconds and is then allowed to cool to a dull red heat in air: the required oxide film is formed during this cooling period. As soon as the nickel has cooled sufficiently, it is sealed to the glass, using an oxidising flame, at a temperature of 650 C. to 750 C.

What is claimed is:

1. A glass for sealing to nickel and to metals and alloys having thermal expansion characteristics such that the mean coefficients of thermal expansion over the temperature ranges of 20 C. to 450 C. and 20 C. to 500 C. do not differ from those of nickel over the same temperature ranges by more than x10" cm./cm./ C., which has a composition, by weight, in the range of 45% to 50% silica (SiO 12% to 22% sodium oxide (Na O), O to potassium oxide (K 0), to 30% alkaline earth metal. oxides (BaO+SrO+CaO) including only 0. to 5% calcium oxide, 0 to 2% fluorine (F 0 to 2% alumina (A1 0 0.5% to 10% zinc oxide (ZnO), and 0.5% to 3% of one of the oxides selected from the group consisting of zirconia (ZrO and titania TiO the total content of alkali metal oxides (Na O-i KiO) being in the range of 18% to 22%..

2..v A glassaccording to. claim 1. which is" composed of 48.0% SiO 21.0% Na O, 8.5% BaO, 10.0% SrO, 3.0%v CaO, 1.5% P 1.0%, A1 03, 5.0%.. ZnO,. and 2.0% ZrO by weight.

3.. A glass according to claim 1 which. is composed of 47.0% SiO 20.5% Na O, 9.5% BaO, 10.0% SrO, 4.0% C210, 15% P 1.0% A1 0 5.0% ZnO, and. 1.5% ZrO by weight.

4. A glass according to claim 1 which is composed of 50.0% SiO 13.0% 1121 0, 8.0% K 0, 8.0% BaO, 10.0% SrO, 3.0% CaO, 1.0% A1 0 5.0% ZnO,. and 2.0% ZrO by weight.

5. A glass according tov claim 1 which is composed of 50.0% SiO 13.0% Na O, 8.0% K 0, 8.0% BaO, 10.0% SrO, 3.0% CaO, 1.0% A1 0 5.0% ZnO, and 2.0% TiO by weight.

6. A glass according to claim 1 which is composed of, in parts by weight, 48.0 SiO 20.5 Na O, 9.5 BaO, 10.0 SrO, 3.0 CaO, 2.0 F 1.0 A1 0 5.0 ZnO, and 2.0 ZIOZ.

7. A vacuum-tight seal between nickel and glass as set forth in claim 1.

8. A vacuum-tight seal between glass as set forth in claim 1 and a metallic body of thermal expansion characteristic such that the meancoefficients of thermal expansion over the temperature ranges of 20 C. to 450 C. and 20 C. to 500 C. do not differ from those of nickel over the same temperature ranges by more. than 0.5 10 cm./cm./ C.

References Cited in the file of this patent UNITED- STATES PATENTS 2,3l8,435 Stupakoif et al May 4, 1943 ,386,685 Hood Oct. 9., 1945 2,429,432 Stanworth Oct. 21, 1947 2,454,607 Leberknight et al Nov. 23, 1948 2,468,868 Danzin et al May 3, 1949 2,587,914 Smith Mar. 4, 1952 2,634,555 Henry et al. Apr. 14, 1953 2,667,432 Nolte Jan. 26, 1954 2,670,572 Smith Mar. 2, 1954 2,688,559 Armistead Sept. 7, 1954 2,693,423 Rogers Nov. 2, 1954 2,697,309 Gates Dec. 21, 1954 2,720,997 Dailey et al. Oct. 18, 1955 2,730,260 McCullough Jan. 10, 1956 2,753,073 Faulkner July 3, 1956 2,770,923 Dalton et a1. Nov. 20, 1956 2,821,811 Hagenberg Feb. 4, 1958 2,844,919 Power July 29, 1958 

1. A GLASS FOR SEALING TO NICKEL AND TO METALS AND ALLOYS HAVING THERMAL EXPANSION CHARACTERISTICS SUCH THAT THE MEAN COEFFICIENTS OF THERMAL EXPANSION OVER THE TEMPERATURE RANGES OF 20*C. TO 450*C. AND 20* TO 500* C. DO NOT DIFFER FROM THOSE OF NICKEL OVER THE SAME TEMPERATURE RANGES BY MORE THAN 0.5X10-6 CM./CM./* C., WHICH HAS A COMPOSITION, BY WEIGHT, IN THE RANGE OF 45% TO 50% SILICA (SIO2), 12% TO 22% SODIUM OXIDE (NA2O), 0 TO 10% POTASSIUM OXIDE (K2O), 20% TO 30% ALKALINE EARTH METAL OXIDES (BAO+SRO+CAO) INCLUDING ONLY 0 TO 5% CALCIUM OXIDE, 0 TO 2% FLUORINE (F2), 0 TO 2% ALUMINA (AL2O3), 0.5% TO 10% ZINC OXIDE (ZNO), AND 0.5% TO 3% OF ONE OF THE OXIDES SELECTED FROM THE GROUP CONSISTING OF ZIRCONIA (ZRO2) AND TITANIA (TIO2), THE TOTAL CONTENT OF ALKALI METAL OXIDES (NA2O+K2O) BEING IN THE RANGE OF 18% TO 22% 